Phosphorus recovery feed control method

ABSTRACT

Phosphorus values in the form of P2O5 are recovered from phosphate ores reacted with carbon, silica and oxygen-containing gas in a rotary furnace having a vitreous lining held in place centrifugally and wherein placement of feed is controlled to maintain uniform covering of said lining.

United States Patent [7 2] lnventor Walter C. Saeman Hamden, Conn.

[21] Appl. No. 860,198

[22] Filed Sept. 23, 1969 [45] Patented Jan. 26, 1971 [7 3] Assignee Olin Corporation a corporation of Virginia Continuation-impart of application Ser. No. 727,115, Mar. 18, 1968, now Patent No. 3,479,138, dated Nov. 18, 1967, which is a continuation-in-part of application Ser. No. 398,306, Sept. 22, 1964, now abandoned.

[54] PHOSPHORUS RECOVERY FEED CONTROL METHOD 8 Claims, 4 Drawing Figs.

[52] US. Cl 263/52,

[5 l] Int. Cl F27b 7/00 [50] Field of Search 263/33, 52; 236/ l 58 [56] References Cited UNITED STATES PATENTS 2,834,157 5/1958 Bowes 263/33 3,050,018 8/1962 Pearson 263/33X 3,426,968 2/1969 Preisman 263/33 Primary Examiner-John J. Camby Attorneys-Gordon D. Byrkit, Donald F. Clements and Thomas P. ODay ABSTRACT: Phosphorus values in the form of P 0 are recovered from phosphate ores reacted with carbon, silica and oxygen-containing gas in a rotary furnace having a vitreous lining held in place centrifugally and wherein placement of feed is controlled to maintain unifonn covering of said lining.

sis-58.114

PATENIED JAN26 can sum 1 or 2 If INVENTOR ALTER, C. SAEMAN AGENT ATENTED JANZB l97| saw 2 or 2 ANGULAR FURNACE 0$/r/0/v SIGNAL CON TROL COMPUTE TRIGGER/N6 SIGNAL AIR PRESSURE SIGNAL TO A IR CYLINDER FIG 13 R CYLINDER f2 VENTOR WALTER c, SAL-MAN r I f BY AGENT PHOSPHORUS RECOVERY FEED CONTROL METHOD PRIOR APPLICATIONS This application is a continuation-in-part of copending application, Ser. No. 727,115 filed Mar. 18, 1968, now U.S. Pat. No. 3,479,138 issued Nov. 18, 1969, which is in turn a continuation-in-part of application U.S. Ser. No. 398,306 filed Sept. 22, i964, now abandoned. The disclosures of these parent applications are incorporated herein by this reference as fully and completely as if literally copied herein.

This invention relates to pyrochemical processing of raw materials in a fumace' and more particularly to the recovery of phosphorus values from phosphate ores.

This invention has as one of its objects provision of im proved processing of phosphate ores in a centrifugally rotating furnace.

Another object is a new and improved method of processing phosphate ores for efficient and economic production of P directly from phosphate rock. By this method, the cost per ton of P 0 is considerably less than by prior art processes.

More particularly, this invention relates to especially advantageous modifications and improvements in the process of the prior applications recited above whereby the life of the vitreous lining is materially extended and production of phosphorus values is substantially increased. This significantly improved operation is achieved by maintaining a uniform bed of feed covering the vitreous lining in the kiln which avoids local overheating and perforation of the vitreous lining. Suitable means for maintaining the uniform feed covering include temperature sensing means in the bed of particulate refractory underlying the vitreous lining, angular kiln position sensing means, computer means coordinating signals from said temperature sensing means and said angular kiln sensing means and emitting activating signals to feed means thereby delivering an increment of feed to the area of highest temperature in said feed bed.

In conventional electric furnaces and blast furnaces, heat generation and heat distribution are not uniform, principally because the charge materials are poor heat conductors and convection is inadequate. The operation of the present invention avoids these difficulties, improves the efficiency of the operation, effectively reduces dust fonnation and results in a process stream richer in the phosphorus productffhe process of the present invention is carried out in an open cavity furnace in which the charge is fused. The desired products are vaporized and removed from one end of the furnace while slag is removed from the other end of the furnace. Radiation of heat promotes good heat transfer, suppressing dust formation and, surprisingly, affords means for the oxidation of phosphorus to its pentoxide and CO to CO greatly increasing fuel economy.

Klugh, in US. Pat. No. l,492,7l3 maintains a molten slag as reaction medium, heats it by immersing arc electrodes therein and charges the reactants into the molten slag.

Lapple, in US. Pat. No. 3,241,917 maintains a reaction mixture of pulverulent reactants and products, covers the reaction mixture with an overlay of free coke to isolate the feed from the oxidizing atmosphere and to insulate the reaction mixture from the radiant heat of the flame. Lapple also limits the temperature to i500 C. (2732 F.) to avoid sintering and formation of a fluid slag. In Lapples furnace, twothirds or more of the refractory insulation is directly exposed to the hot gases at flame temperatures and to the P 0 and fluorine compounds contained therein. The resulting heat and corrosion are seriously deleterious to the life of the kiln. Residence times in hundreds of minutes are required compared to residence times of about 1 to 30 minutes in the process of the present invention.

ing is material of the same kind as that being processed. Rotation of the furnace generates centrifugal force holding a substantial layer of unbonded granular thermal insulation on the inner wall of the furnace shell. Temperature is maintained to fuse the inner surface of the lining material to form the walls of a bowl of fused material adhering smoothly to the underlying unbonded granular material.

US. Pat. No. 3,030,094 shows a rotary fusion furnace in conjunction with a rotary preheat furnace. The fusion furnace is fed at one end and heated by a burner at the opposite end where means are provided for removal of the fusion product.

The rotary furnaces of US. Pat. No. 2,878,004 are of particular advantage in the chemical reduction of phosphate ores using carbonaceous reducing agents to recover the phosphorus values of the ores as phosphorus pentoxide. However, certain problems arise in maintaining a covering of feed on the vitreous lining and the solution of these problems forms the basis of the present invention.

According to the invention of the prior applications identified above, phosphate ore, for example, phosphate rock, is processed in a kiln having a vitreous liner maintained cen trifugally on the walls of an elongated furnace cavity in a temperature range of 2800 F. to 4200 F. using carbon as a reducing agent. The evolved phosphorus and carbon monoxide are oxidized immediately in an oxidizing atmosphere to diminish fuel requirements and to produce phosphorus pentoxide and carbon dioxide.

According to the present invention, a uniform bed of feed is maintained on the vitreous lining by the steps of:

1. Providing temperature sensing means in said bed of particulate refractory and producing temperature signals from said sensing means, analyzing said temperature signals and determining the area having the highest temperature above the fusion temperature of said slag;

2. Providing angular kiln position sensing means and producing angular kiln position signals therefrom;

3. Coordinating said temperature signals and said angular kiln position signals and thereby activating feed means to deliver feed to said area having the highest temperature above the fusion temperature of said slag.

Advantageously, the recited means for carrying out the process of the invention are implemented by computer wherein:

1. said temperature signals are relayed from said sensing means to a computer programmed to analyze said temperature signals and to determine the area having the highest temperature above the fusion temperature of said slag;

2. said angular kiln position signals are relayed to said computer;

3. Coordinating said temperature signals and said angular kiln position signals in said computer; and

4. Emitting activating signals from said computer to said feed means.

The furnace is first charged with particulate refractory which is distributed and maintained centrifugally against the shell of the kiln. Sand is particularly preferred as the refractory and the use of sand is particularly detailed here.

The vitreous liner is formed in the furnace by rotating the furnace shell, firing it internally with fuel and oxidizing gases and charging a suitable refractory in particulate form, preferably sand, at either end. The rotation centrifugally forms a bed of loose sand internally on the walls of the furnace and, as firing continues, the sand fuses on the internal surface of the bed. The firing is diminished until the fused liner solidifies to a vitreous lining appropriately from one-half to two inches thick and preferably about one inch thick along the length of the furnace cavity. Sand is fused suitably at about 3200 F. and vitrified by cooling to about 2800 F. Centrifugal action maintains both the vitreous liner and the underlying loose sand over the entire inner surface of the furnace, except in an exhaust zone.

' A particular merit of the rotary furnace for phosphate reduction lies in using sand to form a loose bed adjacent the furnace shell and a vitreous protective inner liner. Sand is low in cost and in the process of this invention, provides advantageous and inexpensive linings. The formation of vitreous silica liners in rotary furnaces by centrifugal action from a bed of loose sand circumvents the serious spalling and cracking problem normally encountered with dense silica refractories. The thermal conductivity of the loose sand underlying the fused liner is only 10 percent that of carbon and about 25 percent that of fire brick. The silica ismost effective as a refractory in vitreous form on the unbonded granular bed of silica.

While sand is preferred as refractory and vitreous liner for this process, lime and limestone have particular advantages in high fusion temperature, insulating value, low cost and performance. Other oxidation resistant refractories are suitable in particulate form for use in this invention, including calcium, magnesium and aluminum silicates, oxides and carbonates. Examples are lime, limestone, magnesia, dolomite and clays. In using the refractories other than silica, silica is advantageously included in the feed to convert the inner surface of these refractories to a fluid slag which flows along the walls of the cavity. It penetrates the underlying refractory, becom ing poorer in silica and richer in the base of the refractory. The fusion point increases and the temperature decreases progressively until the slag forms an impenetrable, solid layer on the underlying particulate refractory. This seal prevents further penetration of the slag, protecting the shell of the furnace and maintaining the reactants on the inner walls of the lining.

Alternatively, linings of silica mixed with the oxides of calcium, magnesium, aluminum are useful. They are suitably formed from physical mixtures of silica and the oxide or from chemically combined fusions of the oxides in granular form. The proportions of these oxides are chosen to yield mixtures with fusion points in excess of the melting point of silica. Combinations of these oxides are also suitably formed in the furnace by depositing centrifugally a layer of calcium, magnesium or aluminum oxide which is then covered with silica or a fusible silicate of a lower melting point.

The open cavity of the rotary furnace is fired with fuel gas and with an oxygen-containing gas. The latter is suitably air but preferably is oxygen-enriched air which is commercially and cheaply available. Pure oxygen or oxygen diluted with air is also suitable. When air is used, it is advantageously preheated to at least 1200 F. The fuel gas is suitably natural gas or waste hydrocarbon gases of high fuel value. Oil or powdered coal are also suitably used as fuels. In the process of this invention, the carbon monoxide and elemental phosphorus coproducts are used as all or part of the fuel requirement. The total heat of oxidation of carbon to carbon dioxide and of phosphorus to its pentoxide are thus recovered and utilized in the process of this invention.

Having formed the vitreous liner as described, the introduction of phosphate ore, silica and carbon is started. These are added either as separate streams or they are premixed. The additions are intermittent or continuous. The ratios of carbon to ore and silica to lime are carefully controlled. The feed rate is sufficient to maintain a complete cover over the vitreous lining to protect the vitreous liner and the slag from radiant heat from the flame and from the action of the P formed.

Adjustment of feed rate and distribution serves to maintain a continuous covering for the vitreous liner and the slag and prevents the formation of localized hot zones due to overheating bare slag areas by the flame. Such localized hot zones result from too thin a feed layer and cause excessive volatilization of mineral fumes, P 0 vapor is reabsorbed by the slag and the superheated slag may melt the liner, penetrating the underlying particulate refractory. Conversely, too thick a covering of feed on the vitreous liner causes the underlying slag to freeze and form a dam behind which molten slag accumulates. If this slag pool becomes too deep, slag cascading occurs, even though furnace shell rotation is above critical speed. Resulting effects are reabsorption of P 0 vapor by the slag and gas turbulence which mixes the oxidizing and reducing gas zones, thus increasing direct oxidation of carbon to carbon dioxide and reducing phosphorus volatilization and production rates. These problems are avoided by maintaining uniform feed coverage, suitably 0.5 to 6 inches thick, on the vitreous liner.

The improvement of the present invention is directed to maintaining said uniform feed coverage thereby extending the life of the lining and the production time of the kiln between each rebuilding of the'lining.

The carbonaceous material in the feed to the furnace is suitably coke. It is appropriately incorporated as such with silica and phosphate ore in the feed or, alternatively, the feed is prepared using coal, silica and ore, then carbonizing the coal to coke in the mixture before feeding it to the phosphorus furnace. Mixed grades of coke ranging from fines to coarse lumps an inch in diameter and even up to 3 inches in diameter are especially advantageous. The coarser fraction of the coke assures anchorage of these particles in the moving stream of fused slag on the inner liner. This avoids flotation of the coke on the slag and vastly improves reactive contact between the coke and ore. This in turn assures rapid and complete reduction of the phosphorus in the ore. The larger the coke lumps the thicker the layer of fused slag that can be carried without displacement of the coke lumps from the cavity wall. Coke fines are beneficial in that they increase the surface of carbon in contact with the ore while remaining enmeshed among the coarser lumps. Coke fines alone are undesirable since they do not agglomerate as readily and tend to float on and blanket the feed, preventing radiant heat transfer.

Preferably the average particle size of the coke is at least 4 times as thick as the layer of molten slag. The larger particles of coke do not float but rest solidly on the vitreous liner underlying the fused slag. in this manner, the entire inner'surface of the liner is usable'as reactive hearth surface for the coke while the molten slag flows past the coke particles in a thin, continuous sheet.

Because coke fines or breeze are considerably cheaper than lump coke, it is particularly advantageous to utilize all fines. This is accomplished by premixing fine coke with finely ground phosphate ore and agglomerating the mixture into pellets. The highest efficiency is achieved when the feeds are dry and the carbon is premixed with the ore prior to injection into the furnace.

More particularly, the rock and the coke are first preground, substantially all passing a 200 mesh screen, blended and granulated suitably in a mixer with the addition of moisture and a binder after which the resulting pellets are dried to pebbles up to about 1 inch in size. The dry, agglomerated mixture is fed into the reaction zone. The reduction proceeds rapidly and is completed at the reaction temp'erature.

Especially advantageous for reducing heat requirements is the use of powdered coal for feed'preparation, coking the coal in the feed mix and introducing the hot feed into the phosphorus furnace with minimum loss of heat during the transfer.

The theoretical carbon requirement for reducing the phosphorus in phosphate 'to elemental phosphorus is about 1.0 lb. carbon per pound phosphorus. However, to produce the temperatures necessary for the reaction, additional carbon is burned to CO in an oxygen-bearing gas. In order to achieve proper performance the carbon to phosphorus weight ratio is maintained between. about 1:1 to 4:1, preferably 1.6.1 to 1.75:1. This is necessary to supply sufficient carbon to reduce the phosphate values plus an additional increment to provide the carbon which is oxidized.

It is an advantage of the process of this invention that the carbon introduced is suitably burned to carbon dioxide and all of the heat value of the carbon is utilized. It is a further advantage of the process of this invention that the heat of cornbustion of the phosphorus to its 'pentoxide is also conserved.

Varying amounts of silica in the ore contribute to the maintenance of the liner but some silica may be removed during the process in the form of calcium silicate slag. To maintain the liner, supplemental amounts of sand are introduced as a separate stream, admixed with the coke or ore streams but preferably are included in the pelletized feed. The additions of sand as necessary to maintain the lining and the underlying bed, when it is particulate sand, are most advantageously accomplished by the process of the present invention.

Lining perforations, which are avoided using the process of the present invention, generally appear to form in areas where (l) the depth of the bed of feed on the liner is insufficient to protect it from direct radiation of flame temperatures and where (2) the lining is exposed to the action of byproduct ferrophos.

Flame temperatures resulting from the oxidation of the phosphorus and C0 are well in excess of 3140 F., the fusion temperature of silica. However, fused silica at this temperature and at somewhat higher temperatures is very viscous. As a result, physical displacement of fused silica near its melting point occurs only slowly. Pure silica liners .can be exposed to temperatures substantially in excess of 3140 F. in the centrifugally lined rotary furnace with little displacement of the liner in view of the high viscosity of the fused silica. This relatively immobile surface functions as a thermal barrier to prevent the exposure of the underlying strata of the liner to temperatures in excess of the fusion temperature. The silica liner can therefore be retained intact at temperatures in excess of 3140 F.

Ferrophos results from the reduction of impurities in the phosphate ore. It has a high density and, in comparison with the viscosity of fused silica, is extremely mobile. Exposure of the lining to a pool or globule of ferrophos at temperatures in excess of 3 140 F. permits an immediate transfer of this high temperature heat to the interface between the ferrophos and the underlying silica liner due to the high mobility and density of the fused ferrophos. The ferrophos sinks into the softened silica which is progressively displaced. Additional quantities of ferrophos accumulate in the depression until the liner is completely perforated. The ferrophos disperses into the porous sand underlying the fused quartz liner and other liquid reaction products follow until the serviceability of the lining is seriously impaired.

Liner perforations are substantially avoided even at temperatures above 3140 F. by maintaining a complete cover of feed over the entire lining. The bed of feed forms a thermal barrier between the flame and the vitreous liner which avoids temperatures in excess of 3140" F. in the liner. Perforations are thereby avoided and the liner can be operated indefinitely without impainnent by perforation.

Previously, during periods of vigorous reaction, the oxidation of phosphorus in the flame obscured vision and it was necessary to defer, until the furnace atmosphere cleared, any visual inspection and placement of additional increments of feed to cover bare spots. Direction of placement of feed had to be deferred until the feed layer on the lining was largely depleted. Heat losses during periods of operation with insum cient feed bed decreased thermal efficiency. Optical pyrometers offer no advantage over direct visual observation when vision is obscured. Also, thermocouples are only with difficulty located in the reaction interface on the lining and they disintegrate rapidly in contact with the reactants. Problems also arose in directing placement of feed due to rotation of the kiln.

According to the present invention, thermocouples or other temperature sensing means are suitably located in the bed of particulate refractory as closely as possible to the lining. There the thermocouples are out of contact with the reactants and they measure a temperature related through a small temperature gradient and after a small time interval to the temperature at the lining-reaction mixture interface. The measured temperatures are extrapolated spacewise and timewise to deter- However, when the measured temperatures are between 2600 F. and 2800 F., the lining is covered with the optimum thickness of reacting feed, production is at a maximum rate and the lining is protected from exposure to perforating temperatures.

Besides thermocouples, other suitable temperature sensing devices include resistance thermometers, thermistors, bimetallic indicators and the like.

Selective placement of cool feed in areas of highest temperature, above the melting temperature of the slag, covers the thin areas and maintains uniform covering by the feed on the lining. Control of the feed means requires correlation of area temperature with rotational speed of the kiln and any delay between temperature sensing and delivery of feed.

One suitable device for selective placement of feed is the specific embodiment described below. Other feed devices amenable to adequate control are also suitable. The catapult shown can also be modified by substitution of equivalent linkages and other driving devices which are suitably activated mechanically, electrically, hydraulically or by other fluid pressure, for example, air pressure. Catapult action is synchronized with the position of the rotating furnace drum by a synchronizing pulse generated by the drum. Variable pulse delay up to one revolution of the drum then provides for selective feeding of any peripheral sector of the furnace hearth coincident with the setting of the pulse delay adjustment. Suitable devices deliver feed increments as called for to maintain temperatures over the entire lining in the desired range.

In a particularly advantageous mode of operating the process of the invention, a suitable computer is used to analyze the signals from the temperature sensors, to identify the area of highest temperature above the melting temperature of the slag, to assess the peripheral position of the furnace and any delay between the time of temperature sensing and the time of delivery of feed and to emit an activating signal to the feed device resulting in feed delivery in the selected area.

The feed delivered incrementally as described contains excess silica which results in progressive buildup of the liner in the feed zone. This positively maintains lining thickness during productive operation. By varying the position for placement of the feed as necessary, the lining is maintained and local repairs are made. As a further control of cavity shape, the inner surface is readily accessible and is suitably shaped using a mechanical boring bar intermittently to control and equalize local accumulations of less fusible residues in the rock feed.

The silica to calcium oxide weight ratio must be adjusted to be between 0.6:1 to 2.021, preferably from l.0:l to 1.5:1, to insure that the melting point of the slag is low enough to permit removal of liquid slag from the furnace.

The particle size of the feeds can be varied over a relatively wide range depending on the type of feed selected, method of furnace operation, type of carbon used and method of injecting the solids into the furnace. Preferred procedure involves forming an intimately mixed agglomerate from minus 10 mesh ore, coal, and silica and injecting this agglomerate into the furnace to maintain uniform covering of the vitreous liner. Agglomeration can be accomplished either by low temperature, pressure compacting or by balling, either with or without binders, or by elevated temperature techniques where partial melting of the feed materials provides binding action. Size of the particles of agglomerate is suitably from dust up to about 6 inches.

In operation, once the vitreous liner is formed as described above, the feed of ore and carbon is started either as separate streams or as the premixed agglomerates. The ore and carbon react and radiant heat induces volatilization of the phosphorus with carbon monoxide. The first gas mixture of volatilized phosphorus and carbon monoxide is immediately oxidized to a second gas mixture of phosphorus pentoxide and carbon dioxide in the cavity of the rotary furnace. This permits radiant recovery of the resulting heat values and drives the reaction rapidly. Calcium silicate slag is centrifugally removed from the mine temperatures at the interface with satisfactory accuracy. rock and accumulates in the tapping zone at the firing end of the furnace zone where it is removed periodically or continuously and conveniently by a scooplike scraper and/or trough.

Intermittent feeding requires feeders which are massive in size and require the opening of the furnace door to inject the feed. In intermittent feeding, the injection of massive amounts of cold feed drops the furnace temperature abruptly below the reaction temperature and the reaction is temporarily arrested. It does not resume until the feed cycle is completed and the furnace temperature recovers to reaction temperature from stored heat in the furnace or by supplementary fuel. After the feed charge is consumed by the reaction, the feed cycle is repeated. It is a characteristic of intermittent feed that the reaction also proceeds intermittently in time.v

In contrast, incremental feeding injects the furnace feed at a sufficient velocity to reach the remotest boundary of the vitreous liner by velocity and/or gas blast alone. The feed is injected through a small port in the furnace door and in small amounts per unit time. The furnace temperature never drops below the reaction temperature and the reaction is continuously maintained. Continuous incremental feeding is suitably accomplished by injecting the feed in small feed increments at short time intervals which permits maintaining the uniform covering of the lining with feed, uniform reaction temperature and continuous production of product.

By maintaining flame temperatures in the range of 2900 F. to 4800 F. or higher and by rapidly removing the slag from unreacted feed centrifugally, the reaction temperature is maintained at 2800 F. to 3600 F. and an advantageously high rate of heat transfer is maintained between the radiant heat of the flame and the feed. Generally, the heat flux is at the rate of 10,000 Btu/hr./ft. of vitreous liner at a flame temperature of 3200 F., 100,000 Btu at 3800" F. and 350,000 Btu at 4800 F. The feed rate is kept high enough to maintain complete coverage of the vitreous liner and the P production rate is about 1 lb./hr./ft. of vitreous liner at a flame temperature of 3200 F., lb./hr./ft. at 3800 F. and 35 lb./hr./ft. at 4800 F.

Accompanying FIG. 1 shows a cross section of a suitable furnace; FIG. 2A shows a feeding device retracted; FIG. 2B shows the feeding device extended and FIG. 3 shows the computer link between the furnace and the feeding device.

In FIG. 1, rotary furnace is supported on tires 21. End seal 24, connects the furnace to stationary exhaust duct 25. The furnace is loaded with centrifugally retained unbonded sand 22 sealed by impervious fused quartz liner 23 produced by the fusion of the inner surface of the sand liner. The feed end of the furnace is closed by refractory door 26 containing feed port 31. The door is supported by bars 32 shown by solid lines in closed position and by dotted lines in open position for visual inspection and for removing slag by extending scraper channel 29. The scraper channel delivers slag to sluice 30 for transfer to a slag disposal area. Catapult 27 delivers feed from hopper 28 to furnace 20 through port 31 in door 26 as shown in more detail in FIGS. 2A and 28. Members 55 and 56 are elements of the angular furnace position sensing means and consist of contact 55 attached to and rotating with the furnace, suitably an electrically conductive brush, and stationary electrical contact 56. Electric power is supplied to contacts 55 and 56. When contact is made, once per revolution of furnace 20, a pulse flows momentarily via conductors 57 to generate the angular furnace position signal 50 shown in FIG. 3.

In FIGS. 2A and 2B, catapult base 9 is mounted to building structure 10. Parallel link 4 is connected to base 9 through links 6 and 7. Air cylinder 8 is connected to link 7 via shaft 12. Link 3 reciprocates in response to air flow in cylinder 8. Feed cup I is mounted on link 3 which is also attached to link 4. Link 5 connects link 3 with link 7 and holds cup I in vertical position when the catapult is retracted as shown in FIG. 2A and in a position parallel to the trajectory when the catapult is extended as shown in FIG. 2B. When the catapult is retracted, shutter Ila is pushed aside by cup I and the cup is filled with feed. When the catapult is extended, spring Ilb closes shutter 11a stopping the flow of feed from hopper 28 and feed cup 1 projects through feed port 31 in furnace door 26. The catapult thereby receives discrete increments of feed from hopper 28 and injects them through feed port 31 in furnace door 26.

FIG. 3 shows three of a plurality of thermocouples illustrating excess feed at A, normal feed at B and deficient feed at C. Thermocouple 41 registers a temperature 2500 F. at telemetering transmitter 42; thermocouple 44 registers a temperature of 2600" F. at telemetering transmitter 45 and thermocouple 47 registers a temperature of 2700 F. at telemetering transmitter 48 although the flame temperature above each locality is 3400 F. This is due to different thicknesses of feed 40 on lining 23. Commercially available telemetering links coupling the transmitters and receivers 43, 46 and 49 by radio frequency signals are adequate for this service. Temperature signals from all thermocouples in the furnace as well as an angular furnace position signal 50 are combined in control computer 54. The computer is programmed to analyze the temperature data and to identify the area having the highest temperature, corresponding to the greatest deficiency of feed on the liner and to emit triggering signal 52 and air pressure signal 53 to air cylinder 8 (FIG. 1) which activate the catapult at a time and at a rate to effect placement of feed selectively on the area of highest temperature.

In an alternative method avoiding computers, suitably trained personnel analyze the temperature signals, identify the area having the highest temperature, incorporates the appropriate variable pulse delay adjustment with angular furnace position signal 50 and initiates triggering signal 52 and appropriate air pressure signal 53. to activate the feed means and to deliver an increment of feed to the area of highest temperature.

The thermocouple readings are suitably taken to control the feeding device directly. Alternatively, they may be corrected, if desired, by calibration, to correct for (l) variation in the total thermal barrier between the flame and the thermocouple, the total thermal barrier consisting of that of the vitreous liner and the intervening sand insulation and (2) the time lag between the time of variations in the thickness of the feed and the time the associated temperature variations are registered on the thermocouples. By'computer analysis these corrections are added to each of the thermocouples individually to provide a measure of the temperature at the lining-reaction mixture interface.

The operation as describedadvantageously uses feed increments of constant weight or volume but the computer can alternatively be programmed to call for larger or smaller increments based on differences between measured and optimum temperatures in the furnace.

EXAMPLE I A rotary furnace with a shell diameter of 40 inches was driven at revolutions per; minute to maintain a particulate sand lining in contact with the shell and to maintain the reactants and the slag resulting from the thermal reduction of phosphate ore with carbon in contact with the sandlining. The furnace was fitted-with l6 thermocouples mounted in the furnace shell and projecting into the sand lining to a position approximately coincident with the 2700 F. isotherm in the lining. The l6 thermocouples were spaced at four uniform axially incremented peripheral positions of the lining so that each thermocouple signal was related to the localized temperature in one-sixteenth fractions of the total furnace hearth area.

Prior to the injection of feed into the furnace, the lining was heated to 3l00 F. for a sufficient time to establish a steadystate temperature gradient through the lining. The lining temperature was measured accurately with an optical pyrometer. and when it was 3100 F., the average thermocouple reading was 2700 F. Feed was introduced uniformly over the surface of the lining maintaining the average thermocouple reading at 2600 F. When a thermocouple temperature increased above 2600" F., indicating a thinning of the layer of feed on the hearth, the computer directed the catapult to deliver an increment of feed to that area to a rr est the temperature rise and to maintain a temperature of 2600 F. as measured by the thermocouple. Substantially uniform temperatures were thus maintained over the entire area of the hearth. The protective cover of feed on the hearth never became totally depleted and the hearth was never exposed to perforating flame temperatures. The furnace operation continued for more than 50 hours without liner failure. The percentage of useful production time 83.3 percent, allowing a 10 hour shutdown for liner replacement at the end of 50 hours.

in contrast, the same furnace was previously operated using the same feed placed by means of a manually operated feeding arm through the open door of the furnace and using an optical pyrometer to detect high temperature locations on the lining. When the lining was prepared and the temperature of the lining was 3100 F., feed was introduced to form a layer 0.5 to 1 inch deep. For 30 seconds after injection, the furnace atmosphere remained clear and the furnace cavity temperature was quenched. As the temperature ofthe cold feed increased to 2700 F., the furnace atmosphere was obscured by dense fumes and an extremely brilliant, white flame of burning phosphorus at temperatures up to 3600 F. Visibility of the layer of feed on the furnace hearth was completely obscured. After 2 minutes the atmosphere near the leading edge of the feed zone cleared due to complete depletion of the feed layer. Propagation of this zone of clear atmosphere continued for l to 3 minutes to the trailing edge of the feed zone until all of the ore feed previously introduced was consumed. During this period of propagation, the hearth at the leading edge of the feed zone was exposed to flame temperatures in excess of the fusion temperature of the liner. in place of the layer of the ore feed, there remained a molten layer of calcium silicate slag uniformly distributed on the hearth and draining to the tapping zone of the furnace. At this time optical pyrorneter temperature readings were takenand used to adjust the auxiliary fuel firing rate to maintain an operating temperature of 3 100 F. Intermittent injection of cold feed covering the entire lining was repeated. in this intermittent method of operation a major portion of the lining, particularly in the forward part of the reaction zone and in the slag tapping me was exposed to temperatures in excess of the fusion temperature of the silica lining for a major portion of the feed cycle. Resulting liner perforations containing ferrophos led to liner failure after 6 hours of operation. Liner restoration required l hours. The productive time was six-sixteenths or 37.5 percentcompared with 83.3 percent using the process of the present invention.

Among the special advantages of the process of the present invention are:

1. Continuous monitoring and measuring the thickness of the layer of feed on the furnace hearth.

2. injection of the fumaoe feed through an axially positioned orifice in the furnace door, selectively depositing the feed on specific areas of the lining.

3. Automatically controlling the selective placement of feed to maintain an essentially uniform and continuous cover of feed on the lining.

4. Selective placement of feed on the lining to minimize quenching of the phosphate reducing reaction by cold feed.

5. Maintaining the productivity of the furnace and the efficiency of carbon and ore utilization.

6. Protection of the lining from perforating temperatures to extend liner life.

I claim:

1. In the thermochemical process for recovery of P 0 from phosphate ores wherein a feed of silica, carbon and the phosphatic ore is heated to produce a slag and a first gaseous 5 mixture of P and CO; said first gaseous mixture is heated with an oxygen-containing gas to form a second gaseous mixture of P 0 and CO and P 0 is separated from said second gaseous mixture; and wherein: (l) a bed of particulate refractory is distributed and centrifugally maintained in a rotating kiln; (2)

a centrifugally maintained vitreous lining on said refractory is formed by fusing the inner surface of sai refractory and cooling the resulting fused surface; (3) said feed is distributed and centrifugally maintained on said lining; (4) oxygen-containing gas and fuel is burned in said kiln to produce heat and flame, thereby maintaining the reaction temperature between about 2800 F. and 4200 F., forming said slag and said first gaseous mixture and converting said first gaseous mixture to said second gaseous mixture; (5) said slag is centrifugally transferred from said feed to said liner thereby maintaining radiant heat exchange between said flame and said feed; and (6) said slag and said second gaseous mixture are removed as separate streams from said kiln and said P 0 is separated from said second gaseous mixture; the improvement of maintaining a uniform distribution of feed covering said lining and thereby extending the life of said lining by the steps of:

l. Sensing the temperatures in said bed of particulate refractory and producing corresponding temperature signals, analyzing said temperature signals and determining the area of said bed having the highest temperature;

2. Sensing angular kiln positions and producing angular kiln position signals therefrom;

3. Coordinating said temperature signals and said angular kiln position signals to produce a coordinated signal; and

4. Generating from said coordinated signal an activating signal to feed means whereby said feed means is activated to deliver feed to the area of said bed having the highest temperature.

2. Method as claimed in claim 1 in which:

i. said temperature signals are analyzed and the area of the bed having the highest temperature is determined by relating said temperature signals to a computer programmed to identify said area;

2. relaying said angular kiln position signals to said computer; and

3. activating feed means to said kiln by coordinating said temperature signals and said angular kiln position signals in said computer and emitting activating signals from said computer to said feed means.

3. Method as claimed in claim 1 in which said temperatures 50 are sensed by means of thermocouples embedded in said bed.

4. Method as claimed in claim 3 in which said thermocouples are embedded in said bed of particulate refractory adjacent said vitreous lining.

5. Method as claimed in claim 4 in which said thermocou- 5 5 ples are uniformly distributed in said bed of particulate refractory to provide temperature sensing in uniform segments of the area of said lining.

6. Method as claimed in claim 1 in which said feed is delivered to said bed using a catapult.

0 7. Method as claimed in claim 6 in which said catapult is fluid-activated and said activating signal is an increment of fluid pressure.

8. Method as claimed in claim 7 in which said fluid is air. 

2. Sensing angular kiln positions and producing angular kiln position signals therefrom;
 2. Method as claimed in claim 1 in which:
 2. relaying said angular kiln position signals to said computer; and
 3. Method as claimed in claim 1 in which said temperatures are sensed by means of thermocouples embedded in said bed.
 3. activating feed means to said kiln by coordinating said temperature signals and said angular kiln position signals in said computer and emitting activating signals from said computer to said feed means.
 3. Coordinating said temperature signals and said angular kiln position signals to produce a coordinated signal; and
 4. Generating from said coordinated signal an activating signal to feed means whereby said feed means is activated to deliver feed to the area of said bed having the highest temperature.
 4. Method as claimed in claim 3 in which said thermocouples are embedded in said bed of particulate refractory adjacent said vitreous lining.
 5. Method as claimed in claim 4 in which said thermocouples are uniformly distributed in said bed of particulate refractory to provide temperature sensing in uniform segments of the area of said lining.
 6. Method as claimed in claim 1 in which said feed is delivered to said bed using a catapult.
 7. Method as claimed in claim 6 in which said catapult is fluid-activated and said activating signal is an increment of fluid pressure.
 8. Method as claimed in claim 7 in which said fluid is air. 